1. Field of the Invention
The present invention relates to positive characteristic thermistor devices made of semiconductor ceramic materials.
2. Description of the Related Art
Conventional positive characteristic thermistor devices (i.e., positive temperature characteristic devices having a positive temperature coefficient, or "PTC devices") include a structure as shown in FIG. 1. This positive characteristic thermistor device 1 is formed by providing electrodes 3 on opposite sides of a device main body 2 made of a substantially uniform semiconductor ceramic material, and electrically connecting a lead wire 4 to each of the electrodes 3 by means of soldering or like technique. Such a PTC device is used for various applications including protection of a circuit against excess current flowing in the circuit (referred to hereafter as an "overcurrent") because of the fact that its resistance abruptly increases at a temperature equal to or higher than the Curie point. Specifically, when an overcurrent flows through the PTC device, the temperature of the PTC device abruptly increases which in turn greatly increases the resistance of the device. This cuts off the current to the circuit in which the PTC device is inserted, thereby protecting the circuit against the overcurrent.
A conventional PTC device also exhibits a self-resetting property as a protection measure, wherein the PTC device shorts due to erroneous wiring resulting in application of an excessive voltage (hereinafter referred to as "overvoltage") on the order of 200 V. The PTC device returns to its initial state when the overvoltage is removed, which eliminates the need for replacing the PTC device.
When a voltage is abruptly applied through the lead wire 4 to the PTC device 1 as shown in FIG. 1, the device main body 2 generates heat. FIG. 2 shows the result of a measurement made using an infrared temperature analyzer of the temperature distribution in the PTC device during the generation of heat at the time of energization. In FIG. 2, the temperature distribution in the PTC device 1 is illustrated using isothermal lines 5. As shown in FIG. 2, the temperature is higher in an inner region of the PTC device 1 and lower at the surface of the device. As a result, when a voltage is abruptly applied to the PTC device 1, breakage can occur due to thermal stress originating from the temperature difference between the inner region and the surface of the device.
A close study of this breakage phenomenon due to thermal stress led the present inventors to the following insight into the breakage mechanism of the device. When a voltage is abruptly applied to the PTC device, heat is generated in the PTC device by the current that flows therethrough. The temperature becomes higher in an inner region of the device than in a surface region thereof due to a difference in heat dissipation properties between the inner and surface regions of the device. If the temperature is higher in the inner region of the device, the inner region of the device will have a resistance higher than that of the surface region. This further increases the amount of heat generated in the device inner region. The temperature difference between the inner and surface regions of the device increases because of their different heat dissipating properties and the increase in the resistance of the device. A resultant difference in the thermal expansion properties between the inner and surface regions of the device leads to breakage of the PTC device.
Because of the potential for breakage due to thermal stress as described above, a circuit is sometimes protected due to the breakage of the PTC device when an overvoltage as high as 600 V is applied to the PCT device. That is, the breakage creates an open-circuit which prevents damage to the circuit. However, when a conventional PTC device is broken by an overvoltage on the order of 600 V, the breakage of the device main body often is such that the device main body is cracked rather than being completely broken. If a PTC device is cracked instead of being completely broken (such a mode of breakage is hereinafter referred to as "insufficient breakage"), sparks occur at the cracked regions, resulting in a short circuit in the PTC device. This causes a very high overcurrent to flow through the circuit when the device is used, for example, as a component for protecting a circuit from an overcurrent. This can lead to critical accidents, e.g., a short circuit of the terminal equipment and damage resulting therefrom.
A current fuse can be used instead of a PTC device, but current fuses have their own disadvantages. More specifically, a current fuse blows out upon the application of excess current and voltages and does not have a self-resetting property. That is, a current fuse operates by blowing out even upon the application of an overvoltage on the order of 200 V and, in each of such blow outs, the current fuse must be replaced. This has been inconvenient due to the troublesome maintenance operations that must be carried out.
It is an exemplary object of the present invention to solve the above-described problems, and more specifically to provide a positive characteristic thermistor device capable of reliably and quickly cutting off a current to produce an open circuit when overvoltage is applied thereto.
A positive characteristic thermistor device according to a first aspect of the invention includes a device main body having a multi-layer structure including three or more semiconductor ceramic layers. The device main body includes a ceramic layer having relatively high porosity sandwiched between ceramic layers having relatively low porosity.
In this positive characteristic thermistor device, the ceramic layer having relatively high porosity is sandwiched between the ceramic layer having relatively low porosity. Therefore, when a high overvoltage is applied to the device or a high overcurrent flows through the device, the heat generated in the ceramic layer of higher porosity (having higher resistance) is higher than the heat generated in the ceramic layers of lower porosity (having lower resistance). This results in a difference in the degree of thermal expansion between the ceramic layer of higher porosity and the ceramic layers of lower porosity. As a result, thermal stress develops in these regions, which causes delamination (that is, breakage) of the positive characteristic thermistor device near the ceramic layer of higher porosity.
Further, since the ceramic layer of higher porosity is lower in strength, it is more prone to delamination when an overvoltage is applied thereto or an overcurrent flows therethrough. This allows the positive characteristic thermistor to reliably enter a non-conductive state to eliminate the possibility of insufficient breakage when an overvoltage is applied to or an overcurrent flows through the positive characteristic thermistor device.
A positive characteristic thermistor device according to a second aspect of the invention includes a device main body made of a semiconductor ceramic material which has a region having porosity higher than that of neighboring regions.
In the positive characteristic thermistor device according to the second aspect of the invention including a region having porosity higher than that of its neighboring regions, when a high overvoltage is applied to or a high overcurrent flows through the positive characteristic thermistor device, a disproportionate amount of heat is generated in the region of higher porosity. Consequently, thermal stress develops between the high porosity region and the neighboring regions. This causes delamination in the positive characteristic thermistor device. Further, the region having higher porosity (which is surrounded by the neighboring regions of lower porosity) radiates heat poorly, which promotes the development of thermal stress and consequently delamination of the positive characteristic thermistor device. Moreover, the region having higher porosity is lower in strength, which further promotes delamination. Thus, the positive characteristic thermistor device according to the second aspect of the invention can also reliably enter a non-conductive state when an overvoltage is applied thereto or an overcurrent flows therethrough to eliminate the possibility of insufficient breakage.
A positive characteristic thermistor device according to a third aspect of the invention includes a device main body made of a semiconductor ceramic material having porosity continuously varying from a surface region thereof toward an inner region thereof. Further, the device main body includes a region having relatively high porosity in which the varying porosity exhibits a maximum value.
The positive characteristic thermistor device according to the third aspect of the invention including a region having a maximum porosity also provides delamination in the region of the maximum porosity due to thermal stress caused by generation of heat in the ceramic layer having the maximum porosity when a high overvoltage is applied thereto or a high overcurrent flows therethrough. Moreover, the region having higher porosity is lower in strength, which further promotes delamination. Thus, the positive characteristic thermistor device according to the third aspect of the invention can also reliably enter a non-conductive state when an overvoltage is applied thereto or an overcurrent flows therethrough to eliminate the possibility of insufficient breakage. The porosity can vary in any of one-dimensional (laminar), two-dimensional and three-dimensional modes.
According to a fourth aspect of the invention, there is provided a positive characteristic thermistor device in accordance with any of the first, second and third aspects, characterized in that the porosity is at a maximum in a portion substantially in the center of the device main body. Providing a maximum porosity in the center of the main body can be achieved by providing a central portion of the device main body having a ceramic layer with relatively high porosity, by providing a region having porosity higher than that of its neighboring regions, or providing a region in which the porosity exhibits a maximum value. Since heat generated in these high porosity regions is difficult to release, thermal stress between these regions and the neighboring regions (e.g. regions on both sides of the high porosity region) is further promoted. This phenomenon more reliably induces delamination of the positive characteristic thermistor upon the application of an overvoltage or overcurrent thereto.